1. Field of the Invention
The present invention relates to glasses and glass-ceramics with high elasticity moduli and their uses.
2. Prior Art
Glass is used as a data carrier substrate for fixed disks instead of metal, such as aluminum or metal alloy, because of its planarity and its reduced surface roughness, among other reasons. It is possible to polish glass, which is a most uniform material, to provide a very smooth surface on a glass body. Also the speed and economy of the glass substrate production process are comparable to the speed and economy of the production process for aluminum substrates.
Substrate glass for fixed disks must withstand large chemical, thermal and mechanical loads. Thus during coating (for example by cathode sputtering) it experiences high temperatures with high cooling rates. In use as a fixed disk substrate high mechanical loads occur, e.g. additional stresses occur in operation at the currently used high rotation speeds of 3,500 to 10,000 U/min because centrifugal forces and clamping stresses of up to 100 N/m.sup.2 occur at the rotation axis during manufacture. Thin glass, especially with a thickness of from 0.25 to 3.00 mm can only stand these stresses when it is pre-stressed. Since the increase of mechanical load resistance by thermal pre-stressing is possible at a minimum thickness of 3 mm, glasses must be chemical pre-stressed for the above-named application. They are significantly pre-stressable by ion exchange in a salt bath under the transformation temperature T.sub.g, i.e. suitable ions, such as K.sup.+ and/or Na.sup.+ and/or Ba.sup.2+, have been shown to be sufficient for exchange with Na.sup.+ and/or Li.sup.+ and/or Ca.sup.2+ ions.
An additional essential property of glass suitable for use as a fixed disk substrate is its thermal expansion behavior, which must not vary too much from that of the coating material (e.g. coalloys with thermal expansion coefficients .alpha..sub.20/300.gtoreq.12.times.10.sup.-6 /K) and above all not too much from that of the clamping material and spindle material of the drive mechanism (with .alpha..sub.20/300.gtoreq.12.times.10.sup.-6 /K) in order to avoid stresses and strains.
Glass-ceramic is an interesting material for the above application above all because of its fracture or breakage resistance. However the crystallite size limits the surface residual roughness to a high value in currently used glass-ceramics.
The developments in the fixed disk market are directed toward data carriers with higher capacity and greater data transfer rates with the dimensions of the data carriers remaining the same or being reduced. Higher data transfer rates require a higher rotation speed for the fixed disk in the drive mechanism. The capacity can only be increased while maintaining the dimensions the same by a higher track density or by an increase in the number on the fixed disk in the drive mechanism. However an increase in the rotation speed causes a strong flutter or pulsation of the fixed disk outer edges, which again makes the desired higher track density, also a reduced track spacing and also a narrow stacking of the fixed disks, impossible. Because of this flutter motion also the flight height or glide height of the read-write head over the fixed disk cannot be reduced, as is desired for an increasing the read/write speed and the information density.
The fixed disk thus requires a high shape stability, i.e. it should have a time dependent disk flutter that is as small as possible at its outer edges. The maximum disk flutter W is given by the following formula I: EQU W={[.rho..times.r.sub.A.sup.4 ]/[E.times.d.sup.2 ]}f(.nu.) I,
wherein
.rho.=density PA1 r.sub.A =outer diameter of the fixed plate PA1 E=elasticity modulus PA1 d=thickness of the fixed plate PA1 f(.nu.)=geometry-specific parameters. PA1 and at least one refining agent, as needed, in an amount suitable for its purpose, namely refining, PA1 with the proviso that the sum of La.sub.2 O.sub.3 +Ce.sub.2 O.sub.3 content present is less than or equal to 16 percent by weight based on oxide content. PA1 and at least one refining agent, as needed, in an amount suitable for its purpose, namely refining, PA1 with the proviso that the sum of La.sub.2 O.sub.3 +Ce.sub.2 O.sub.3 content present is less than or equal to 16 percent by weight based on oxide content. PA1 and refining agents, as needed, in an amount suitable for their purpose.
The chief specifications or requirements for fixed disks can be derived from this formula. When the geometry remains the same (i.e. d, r.sub.A, constant) the maximum flutter W is reduced when the elasticity modulus E is higher and/or the density .rho. is less. Usually the quotient of these parameters E/.rho. is designated the specific elasticity modulus. It should take the highest possible value.
The glasses and glass-ceramics known for this application are chiefly high SiO.sub.2 -containing alumino-silicate glasses or lithium aluminum silicate glass-ceramics, which have poor fusion properties because of their high SiO.sub.2 content and high Al.sub.2 O.sub.3 content. The chemically improved glass composition for substrates for information recording disclosed in DE 42 06 268 A1 having a content of from 62 to 75 percent by weight SiO.sub.2 should be mentioned. Also the glass-ceramic for magnetic disk substrates disclosed in EP 626 353 A1 having a SiO.sub.2 content of from 65 to 83% by weight, which contains .alpha.-quartz and lithium silicate, should be mentioned in this connection.
The known glasses and glass-ceramics and other materials do not fulfill all the requirements for a material for a fixed disk, especially a fixed disk with a high rotation speed at the same time, but have a very wide variety of disadvantages.
Glasses for use as a substrate for plasma display panels are described in JP 9-301732 A. These glasses have a high SiO.sub.2 content and high Al.sub.2 O.sub.3 content and also a high content of alkali oxides, which leads to a reduced E-modulus.
JP 52-45612 A describes glasses with a high content of Nb.sub.2 O.sub.5, which similarly leads to a reduced E-modulus. Also the glasses of DE 34 20 306 C2 contain a high Nb.sub.2 O.sub.3 content.
Similarly the same is true for the glasses disclosed in JP 52-9012 A, which can contain a high content of Nb.sub.2 O.sub.5, while the content of TiO.sub.2 which increases the E-modulus must remain limited to a low level. The same is also true for the glasses of DE 26 60 012 C2, in which the content of the facultative component La.sub.2 O.sub.3 additionally remains limited to a small percentage, which also is contrary to obtaining a high E-modulus.
EP 0 287 345 A1 describes glasses for lenses with a refraction gradient, which have a comparatively high SiO.sub.2 and Li.sub.2 O content. Because of the existing facultative and other components the glasses of course can have comparatively high E-moduli, but no suitably high specific E-moduli. This is also true for the high Li-content glasses of JP 63-170 247A for the same application. The glasses are very susceptible to crystallization because of the high Li-content. JP 1-133956 similarly describes glasses for lenses with a refraction gradient, whose composition can vary with many facultative components over a wide range, and which however necessarily contain up to 20 Mol % F.sup.- and a comparatively high Al.sub.2 O.sub.3 content.
High Al.sub.2 O.sub.3 glasses comprising the MgO--Al.sub.2 O.sub.3 --SiO.sub.2 system which have high E-moduli are known from U.S. Pat. No. 3,804,646. These glasses are however not chemically pre-stressable because they are free of alkali and also because they crystallize easily.
A data carrier consisting of a laminated disk of glass and a viscoelastic material is disclosed in WO 96/04651, in which oscillations are damped by the layer of viscoelastic material, for example plastic material, such as synthetic rubber, e.g. silicone rubber, or polyester, polyurethane and/or polyamide. However the manufacture of this material is very expensive and it is also disadvantageous that the viscoelastic material shows fatigue with time (becomes brittle) and then can no longer function to dampen the oscillations. Furthermore the plastic materials used can out-gas when the magnetic layer is precipitated at higher substrate temperatures by cathode sputtering and because of that impairs the quality of the applied layer.